Emitted when user-controlled input that can contain HTML can be passed into to an echo statement.


This could lead to a potential Cross Site Scripting (XSS) vulnerability. Using a XSS vulnerability, an attacker could inject malicious JavaScript and execute any action JavaScript could do. Examples include:

  • Stealing authentication material (e.g. cookies, JWT tokens)
  • Exfiltrate sensitive information by reading the DOM
  • Keylog entries on the website (e.g. fake login form)

Whether this is exploitable or not depends on a few conditions:

  • Is an executable mimetype set? (e.g. text/html)
  • Is the content served inline or as attachment? (Content-Disposition)
  • Is the output properly sanitized? (e.g. stripping all HTML tags or having an allowlist of allowed characters)



$name = $_GET["name"];


function printName(string $name) {
    echo $name;


  • Sanitize user-input by using functions such as htmlentities or use an allowlist.
  • Set all cookies to HTTPOnly.
  • Consider using Content Security Policy (CSP), to limit the risk of XSS vulnerabilities.
  • If user input itself is HTML, see Sanitizing HTML User Input

Further resources